Do you want to learn a little more re buttonholes?
The #3 and 3A feet give you the ability to make a larger buttonhole, do corded button holes and make a manual buttonhole, if that is your preference.
The channels in the feet make it easier for cord to be inserted. This is great for bulky and stretch fabrics.
The manual buttonhole element enables you to make a large buttonhole than the 3cm one provided on the 3A and 3B feet.
Here is a little more on the foot.